1. Field of the Invention
The invention relates to a solder joint, and more particularly, to a solder joint with a multilayer intermetallic compound structure between the solder and the Cu pad.
2. Description of Related Art
Soldering is a metallization process to electrically/mechanically join two separate metals with a solder that possesses a relatively low melting point to that of the joined metals. Solders utilized in electronic packaging are mostly the tin (Sn)-based alloy, such as Sn—Pb alloy, Sn—Zn alloy, Sn—Bi alloy, Sn—In alloy, Sn—Sb alloy, Sn—Cu alloy, Sn—Ag alloy, Sn—Ag—Cu alloy, and Sn—Ag—Bi alloy. A common solderable metal (alternatively termed metallization pad) might contain Cu, Co, Ni, P, V, Pd, Pt, Au, Ag, Fe, Zn, or a composite structure thereof.
In general, microelectronic solder joints are fabricated via a wave/reflow soldering process, where a liquid-solid reaction between the Sn-based solder and the metallization pad occurs and an intermetallic compound(s) (IMC) is therefore produced at the solder/pad interface. The Intermetallic compound(s) formation at the interface is a good indication of metallurgical bonding and wetting; however, a thick intermetallic compound layer present at the joint interface is very detrimental to the mechanical reliability of solder joints because intermetallic compounds are all brittle in nature. Additionally, the microstructure and the type of the intermetallic compound might also play a dominant role in the mechanical properties of solder joints. Therefore, modification of the interfacial microstructure to enhance the mechanical characteristics of solder joints is an important reliability issue in the microelectronic packaging.